Recommend using actions/cache in GitHub Actions documentation

[detohm]

Summary

Update the document of Github Actions to add cache action step for reducing the build times. Please refer to the original document of jekyll-action. The cache step is also included for performance improvement.

Therefore, I think this change would benefit all bloggers who are using or begin to use Github Actions to publish thier sites.

There are changes below,

• Update example of workflow .yml code
• Update code description

Context

[ashmaroli]

Thank you @detohm
@jekyllbot: merge +doc